Recently, as base films for transparent electrically conductive films such as EL electrodes, display base plates for liquid crystals, etc., protection films for polarizing films, optical cards, etc., a thermoplastic resin film having a small double refraction and optical isotropy has been used. The reason is as follows. If a thermoplastic resin film having a large double refraction is used, the film is colored according to the phase difference of each wavelength of light, and in the case of using the film for a liquid crystal display plate, the images formed become indistinct. Also, if there are portions of a small double refraction and portions of a large double refraction in the range of the whole film (i.e., there is an uneven distribution of double refraction), uneven color and uneven density occur.
Hitherto, as one of the processes for producing thermoplastic resin films having a small double refraction (and optical isotropy), a flow stretching process has been used. The flow stretching process is a process of flow-stretching (or casting) a solution of an aromatic polyester resin (polyarylate), as the raw material for a film, dissolved in a solvent on a base material such as a resin film (e.g., polyethylene terephthalate film), a metal belt, a metal drum, etc. After subjecting the flow-stretched film to primary drying at a definite temperature to impart a self-supporting property to the film, the formed film is peeled off the base material and subjected to secondary drying until the solvent remaining in the formed film is usually below about 5% by weight to provide a desired synthetic resin film. The reason for drying the film in two stages as described above is that if the formed film is completely dried in one stage, it becomes difficult to peel the formed film off of the base material.
However, in the foregoing conventional process, there is a problem that the retardation value of the synthetic resin film obtained is usually about 60 n.m., and even when a synthetic resin film having a small retardation value is obtained, the double refraction of the film is largely distributed in the width direction and when the film is used as an optical film, the phase axis is diverged.
The reason that the distribution of the double refraction occurs in the width direction is assumed to be that since in the foregoing process, the secondary drying of the film is carried out in a tenter (a drying machine while stretching the film in the width direction) while fixing both edges of the film, when the film is shrunk by heating during drying or the film is shrunk during cooling, a large amount of shrinkage occurs in the central portion of the film and forms strain in the film.